Silicon Thin Films Solar Cell: Its Recent Development

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Silicon Thin Films Solar Cell Its Recent
Development

• Swati RayEnergy Research Unit,
• Indian Association for the Cultivation of
  Science,Jadavpur, Kolkata,
• India.
• E-mail ersr_at_iacs.res.in

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Introduction

• The primary objective of PV solar cell research
  and development is to reduce the cost of PV
  modules and systems to a level that will be
  competitive with conventional ways of generating
  electric power.
• At present Solar cell used for large scale
  terrestrial use is generally made up of
  crystalline Silicon. Wafer based c-Si solar cells
  have relatively high efficiencies (12 - 16
  module efficiency).
• These cells have already proven their excellent
  stability and reliability. The main disadvantage
  associated with the technology is high module
  price.
• Attempts are being made to reduce the cost of
  c-Si cell by using thinner wafers but still there
  are many difficulties.
• Development of thin film silicon solar cell
  technology perhaps is the only option.

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PV Module Production 19902007
1990 47 MW
2007 3700 MW
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Thin film market growth
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The advantages of silicon films

• The fabrication technology is relatively simple
  and inexpensive.
• The product and process are free from
  environmental hazards.
• Absorption coefficient of a-SiH is higher than
  that of c-Si. As much less material is required
  for a-SiH cell, they are lighter in weight and
  less expensive.
• a-Si films can be deposited in one step on a
  large area and on a wide range of substrates,
  including flexible, curved and roll-away type.

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The 3 generations
Solar Cell Technology
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Structure of a-Si Solar Cell
Single Junction
Double Junction
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Multijunction Solar cell

• Multijunction structure enhances the efficiency
  of solar cell, since different active layers of
  stacked solar cell absorb wider range of solar
  spectrum.
• Increase of built in field in the tandem cell can
  reduce metastable defect formation and hence
  stability of solar cell increases.

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The materials developed by PECVD at IACS.

• Undoped and doped amorphous silicon films
  (1.7-1.8 eV) (a-SiH, n-type SiH, p-type
  SiH, a-SiFH)
• High band gap alloys of a-SiH, Silicon Carbide
  and Silicon Oxide (1.8-2.2 eV a-SiCH, p-type
  a-SiCH, a-SiOH, n-type a-SiOxH, p-type
  aSiOxH, a-SiCFH etc.)
• Low band gap alloys of a-SiH specially amorphous
  silicon germanium (1.4 to 1.65eV).
• Microcrystalline Si thin film and its alloys ( µ
  C-SiH, n-type p-type, µ C-SiC, µ C-SiCFH, n
  and p-type , undoped µ C-SiOxH ).
• Nanocrystalline and Protocrystalline silicon.
• Transparent conducting oxides (ZnO, SnO2, ITO)
  by magnetron sputtering method.

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Status of Efficiencies for Si Thin-Film Solar
Cells at IACS, India
Structure Area Efficiency
Degradation (cm2) (Initial) Single
junction 1.0 10.8 20 -
25 Double junction 1.0
10.7
15 Double junction 4 8.0
stabilised (a-Si/ a-Si)
(Measured at NREL, USA) Triple junction
1.0 9.5
13 (a-Si/a-Si/a-SiGe) Single junction
760 7.5 module
(active area) Double junction
760 7.8 module
(active area)
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Performance of single junction a-Si solar cell (1
cm1 cm)
Quantum Efficiency
Current Voltage characteristics
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Spectral response of double junction a-Si solar
cells
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I-V characteristic of double junction a-Si solar
cells
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Multichamber PECVD System
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Laser Scribing System
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Flow Chart of Entire Process
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Double Junction a-Si Module Fabricated in the
Prototype Line
Voc 14.82 V Isc 620.6 mA FF
0.627 Eff 7.59 Power 5.77 W
a-Si modules are fabricated with laser scribing
successively on the TCO, p-i-n and metal surfaces.
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Current Voltage characteristics of 1 ft1ft
module
Quantum Efficiency
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Light induced degradation of single junction
module (Area 30 cm 30 cm)
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Stabilized efficiency and power of solar modules
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Next generation thin film Si solar cell

• Amorphous silicon solar cell technology is in
  commercial stage. R D on a-Si has reached its
  saturation.
• During last few years, attention of the
  scientists in the field of Si thin films are
  drawn towards development of Microcrystalline and
  nanocrystalline based thin film solar cells.
• Double junction cell combining a-Si with mc-Si
  represents very attractive thin film solar cell
  concept.

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a-Si/mc-Si tandem solar cells by KANEKA
a-Si/mc-Si module, 13.5 (4141 cm2 area)
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Present Status of Efficiencies for Si Thin-Film
Solar Cells
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Striking features of microcrystalline silicon
solar cells

• Stability of the efficiency under intense light
  soaking.
• Enhanced extension of the spectral response at
  infrared wavelengths, as compared to a-Si.

Limitations

• Low deposition rate of microcrystalline silicon
  layer.
• Low absorption coefficient compared to a-Si.

Possible Solutions

• High temperature deposition.
• Use of Very High Frequency PECVD.
• High pressure silane depleted RF PECVD.

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Intrinsic micro-Si films

• With increase of HD from 95 to 95.5 ?d
  increases by 4 order of magnitude indicating
  amorphous to microcrystalline transition in the
  silicon network.
• At higher Ts transition occurs at a lower HD.

Fig. Variation of dark conductivity,
photosensitivity at different hydrogen dilutions.
(open symbols for Ts 340oC and closed symbol
for Ts 180oC)
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Roughness (RMS) 8.40 nm
Roughness (RMS) 3.02 nm
AFM of silicon films prepared at 1800C
AFM of silicon films prepared at 3400C
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Deposition Rate

• High rate of nc-SiH depositions requires high
  flux of precursor radicals and sufficient flux of
  atomic hydrogen per monolayer deposition.
• An increase in plasma excitation frequency leads
  to higher power transfer in the plasma, higher
  decomposition of silane, increased ion flux, but
  decrease in electron temperature.
• Higher power increases growth rate but ion
  bombardment may create damage if deposited
  under low pressure.
• High working pressure reduces electron
  temperature of the plasma and avoid damage of the
  film surface.

Fig. Deposition rate of Si films prepared at
different plasma excitation frequencies.
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Structural properties
Fig. Grain size variations of Si films with HD

• lt111gt and lt311gt orientations of c-Si are observed
  in case of high power deposition (gt0.3 W/cm2)
  whereas only lt220gt peak observed at low power.
• The largest grain size is 35 nm using RF as well
  as VHF and the lowest size 5 nm is obtained at a
  chamber pressure of 9 Torr.

Fig. XRD spectra of Si films deposited at
different plasma excitation frequencies.
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Fig. Crystalline volume fraction (Xc)
Fig. Raman spectra of the Si films deposited at
different plasma excitation frequencies.
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54.24 MHz, 50 W
54.24 MHz, 70 W
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Effect of p-layer on i-layer structure
i-layer with nanocrystalline p-layer
i-layer without p-layer
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Light Induced Degradation

• Degradation is significant even after thousand
  hours of light soaking when films is amorphous.
• When the films becomes microcrystalline
  degradation is almost nil but the
  photosensitivity is very low (100).
• For the films deposited at Y 95 98 i.e at
  transition region the films become stable after
  10 hours of light soaking and the
  photosensitivity is reasonably high (100).

Photo conductivities of the Si films after 1000
hours of light soaking
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Fabrication of mc-Si solar cell
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Effect of microstructure on the performance of
single junction microcrystalline silicon solar
cell

• Open circuit voltage is highest at RF-PECVD vhphp
  condition where crystallite size is small.
• Short circuit current is maximum for 105 MHz
  deposited film may be due to less carrier
  recombination at grain boundary.

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Performance of solar cells prepared at 54.24 MHz
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Light induced degradation
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Fabrication of Si quantum dots

• Quantum dots offer the potential to control the
  intermediate band energies.
• Placing the appropriate quantum dot material of
  necessary size into an organized matrix in solar
  cell results in the formation of accessible
  energy levels .
• Theoretically solar cells with quantum dots
  offer a potential efficiency of 63.2.

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Nanocrystalline Si in SiOx matrix
Photoluminesence of SiOx films with different
silicon to oxygen ratio.
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Conclusions

• The low material cost and large area deposition
  capability of amorphous and nanocrystalline
  silicon alloys make this class of materials an
  attractive candidate for cost effective
  production of solar panels. Based on
  announcements by several manufacturers, Industry
  analysts project a production growth from about
  300MW in 2008 to 5000MW in 2012.
• In Japan new NEDO program was initiated in June
  2008,aiming at very exciting target of 40
  including Thin film solar cells towards 2050.New
  program is aiming 30 by 2014 using 5 or 6
  junction thin film solar cells.
• In India, Amorphous Solar cell technology has
  been developed. Work on more stable and efficient
  Nanocrystalline Silicon cell is in progress.
• Extensive research and development work is
  necessary on 2nd and 3rd generation solar cell
  technology to make Solar PV viable for large
  scale use.

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Silicon Thin Film Solar Cells Hokuto, Yamanashi
Roof-top Houses (Kubota Ecolony), KANEKA Hybrid
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Thank you...